ERA-40 precipitation, downwelling radiation and near-surface meteorology were used to force the land-surface model TESSEL for the period 1958-2001. Soil moisture output is used for drought identification and to create a drought index. A verification of the concept is performed over the Iberian Peninsula, where the derived drought index is compared to other indices in use for agricultural and hydrological purposes, such as the standardized precipitation index (SPI) and the Palmer drought severity index (PDSI).

Since precipitation is the major drought atmospheric forcing, a validation of ERA-40 precipitation fields against observations was conducted over Iberia. Despite a dry bias in the rainy season, ERA-40 monthly precipitation explained variance is of the order of 90%, and the 3-, 6- and 12-months SPI are very similar to those from observations.

Normalized total depth soil moisture (NSM) is used as a drought index and compared with the SPI, at several time scales, and the PDSI. The high degree of coherency between the three indices for the 44-year period studied is an indication that NSM can be used as a robust drought indicator. Besides the identification of major drought periods and their intensity, soil moisture was applied to the study of drought spatial and temporal patterns over Iberia. The temporal evolution of soil moisture is related to the accumulated precipitation at various time scales, demonstrated with the correlations of NSM and SPI. PDSI and NSM are related since both indexes are associated with the surface hydrological budget. Soil moisture relaxation times were used to identify areas with longer drought periods (due to scarcer annual precipitation values) where NSM is related to SPI at longer time scales. Drought spatial patterns over Iberia were established from principal components analysis. The spatial heterogeneity of precipitation is the main issue in the spatial analysis of drought, while the spatial distribution of the soil physical characteristics plays a secondary part in the spatial analysis, but is important in the temporal analysis.

The approach to drought analysis used here can be applied to other areas with realistic time variability of ERA-40 precipitation, with no particular model calibration. Forcing from a future near-real time continuation of ERA-40 could be used as a tool in the monitoring of drought situations. Robust early identification of an impending drought could be used to support drought mitigation actions.